Japan's Prime Minister Naoto Kan urged China on Monday to guarantee the safety of Japanese companies and citizens after a wave of rowdy weekend protests sparked by a bitter territorial row. The war of words between the Asian giants, triggered by Japan's arrest last month of a Chinese skipper in disputed waters, showed no sign of abating, with Japan's foreign minister labelling Beijing's reaction as "hysterical". Both nations have sought to arrange a summit meeting later this month to ease their worst spat in years, but weekend street protests in both countries highlighted how the issue has inflamed nationalist passions. Thousands of mostly young Chinese protesters took to the streets of at least four cities to assert China's claim to a disputed island chain where a maritime incident six weeks ago kicked off the heated diplomatic dispute. In protests apparently organised on the Internet and via text messages following an anti-Chinese rally in Tokyo Saturday, thousands of protesters took to the streets, calling for a boycott of Japanese goods. In several of the rallies, demonstrators smashed windows of Japanese businesses, including a Panasonic outlet and an Isetan department store, and attacked Japanese brand cars, news reports in Japan said. The protests, which Chinese authorities struggled to contain, were the largest since 2005, when Japan's then-prime minister Junichiro Koizumi stoked fury by visiting the controversial Yasukuni war shrine in Tokyo. Asia's two biggest economies and traditional rivals have been embroiled in the worst feud in years after Japan arrested a Chinese trawler captain near the disputed islands on September 8, although it later released him. The uninhabited island chain, called Senkaku in Japanese and Diaoyu in Chinese, lies between Japan's Okinawa island and Taiwan, in a part of the East China Sea with rich fishing grounds and thought to contain energy deposits. China has issued a barrage of protests and angry editorials in its state media, while also taking punitive economic steps, including halting the export of rare earths minerals crucial for high-tech products. "I think the countermeasures China has taken are extremely hysterical," said Japan's Foreign Minister Seiji Maehara, known as a China hawk. "As for the rare earth issue, the Chinese commerce ministry says they haven't taken such a measure, but we still can't conclude at this moment that shipments have returned to normal," he told parliament. Beijing and Tokyo have sought to repair their relationship, and possibly hold a summit later this month, but the weekend protests in both countries showed that the incident has reawakend historical animosities. "The government has expressed its regret over the demonstrations against Japan," Kan told parliament, adding that Tokyo has asked "that Japanese nationals and companies be protected". On a conciliatory note, he said Sino-Japanese ties are "a very important bilateral relationship" and that "both sides need to make efforts to handle the situation calmly so as to seek a strategic mutually beneficial relationship." Thousands marched on Saturday in the Chinese southwestern city of Chengdu, the central city of Henan and the ancient capital of Xian. On Sunday, demonstrators gathered in Mianyang in the southwest. "About 10,000 youths marched down the street outside our shop," a woman in Zhengzhou told a news agency. "They were shouting 'Defend the Diaoyu islands' and "Boycott Japanese goods,'" said the woman, who declined to give her name. After the first protests Saturday, Chinese foreign ministry spokesman Ma Zhaoxu said: "It is understandable that some people expressed their outrage against the recent erroneous words and deeds on the Japanese side." But, apparently seeking to calm the situation, Ma added: "We maintain that patriotism should be expressed rationally and in line with law. We don't agree with irrational actions that violate laws and regulations." Japanese media said the protests broke out, mostly in more lightly policed inland cities far from the capital, while political leaders were in Beijing for a communist party congress, possibly catching authorities by surprise.